--- a/src/notes/api/fields/category.py	Tue Jul 10 15:18:03 2018 +0200
+++ b/src/notes/api/fields/category.py	Wed Jul 18 17:32:09 2018 +0200
@@ -1,26 +1,62 @@
 '''
 field serializing and deserializing metacategorization protocols
 '''
+import json
 import logging
 
-from notes import settings
 from rest_framework import serializers
 
+from notes import constants, settings
+from protocols.models import Protocol, ProtocolRevision
+from protocols.serializers import ProtocolRevisionSerializer
+
 logger = logging.getLogger(__name__)
 
 class ProtocolField(serializers.Field):
 
-    def get_attribute(self, obj):
-        logger.debug("ProtocolField.get_attribute %r", obj)
-        # We pass the object instance onto `to_representation`,
-        # not just the field attribute.
-        return obj
+    default_error_messages = {
+        'incorrect_format_str': 'Incorrect format. Expected `'+constants.PROTOCOL_URN_PREFIX+'<ext_id>.<version>`.',
+        'protocol_unknown': 'Incorrect protocol, protocol {ext_id}, version {version} does not exists.',
+        'incorrect_format': 'Incorrect format. Expected `'+constants.PROTOCOL_URN_PREFIX+'<ext_id>.<version>` or dict.'
+    }
+
+    def __init__(self, *args, **kwargs):
+        logger.debug("ProtocolField.__init__ %r || %r", args, kwargs)
+        super().__init__(*args, **kwargs)
 
-    def to_representation(self, obj):
-        logger.debug("ProtocolField.to_representation %r", obj)
-        return settings.NOTES_SETTINGS['PROTOCOL']
+    # def get_attribute(self, *args, **kwargs):
+    #     logger.debug("ProtocolField.get_attribute %r || %r", args, kwargs)
+    #     logger.debug("ProtocolField.get_attribute source %r", self.source)
+    #     # We pass the object instance onto `to_representation`,
+    #     # not just the field attribute.
+    #     return kwargs.get('obj')
+
+    def to_representation(self, value):
+        if isinstance(value, str) and value.startswith(constants.PROTOCOL_URN_PREFIX) :
+            protocol_id, protocol_version = value[len(constants.PROTOCOL_URN_PREFIX):].split('.')
+            # TODO: treat error case, i.e. protocol is not found
+            protocol_revision = ProtocolRevision.objects.get(protocol__ext_id=protocol_id, version=protocol_version)
+            serializer = ProtocolRevisionSerializer(protocol_revision)
+            return serializer.data
+        elif isinstance(value, str):
+            logger.debug("ProtocolField.to_representation value %r" % value)
+            return json.loads(value)
+        else:
+            return value
+
 
     def to_internal_value(self, data):
-        logger.debug("ProtocolField.to_internal_value %r", data)
-        return "base protocol"
 
+        if isinstance(data, str) and data.startswith(constants.PROTOCOL_URN_PREFIX):
+            try:
+                ext_id, version = data[len(constants.PROTOCOL_URN_PREFIX):].split('.')
+            except ValueError:
+                self.fail('incorrect_format_str')
+            if not ProtocolRevision.objects.filter(protocol__ext_id=ext_id, version=version).exists():
+                self.fail('protocol_unknown', ext_id=ext_id, version=version)
+            else:
+                return data
+        elif isinstance(data, dict):
+            return data
+        else:
+            self.fail('incorrect_format')